1. Field
The present disclosure relates generally to testing objects and, in particular, to testing the strength of bonds in objects. Still more particularly, the present disclosure relates to a method and apparatus for testing the strength of bonds in a bonded structure using tension waves.
2. Background
A composite object may be comprised of one or more composite structures that are bonded to each other. Identifying and certifying the strength of bonds may be performed for different applications that use composite structures. These applications include their use in platforms, such as aircraft.
One manner in which bonds may be tested is through the use of laser bond inspection. Laser bond inspection tests the strength of bonds between composite structures within a composite object. In this technique, weak bonds may be “pulled apart” by tension waves traveling through the structure.
With laser bond inspection, a laser beam is directed at the front surface of a composite object. The laser beam creates mechanical waves in the form of compression waves that travel through the composite object toward the back surface of the composite object. When the compression wave reaches a back surface of the object under test, the compression wave is reflected back from that surface producing a tension wave that propagates back toward the front surface of the object. The tension waves apply tension to the internal structure of the object, including any bond lines between the front and back surface of the object. The tension waves may have a sufficient strength that is selected to determine whether bonds between the parts of the object have a desired strength.
Laser bond inspection may be considered a non-destructive testing method when the bonds between composite structures are sufficiently strong. If a tension wave encounters a bond within the composite object that has the desired strength, the bond remains intact and inconsistencies are absent. The composite object may be examined to determine whether any inconsistencies are present in the composite object. If the bond is sufficiently strong, the composite object is not altered and may be used in different applications. This composite object also may be certified as providing a selected strength value.
If the tension wave encounters a bond within the composite object that does not have the desired strength, an inconsistency may occur. If an inconsistency is present, the composite object does not have the desired strength and may be discarded, reworked, or otherwise processed.
Laser bond inspection may require more equipment or cost than desired. Further, the dimensions of the composite object may limit the usefulness of laser bond inspection. For example, both the compression and tension waves lose strength while traveling through the composite object. When testing larger or denser composite objects, the resulting tension wave may not generate as much force as desired. In other words, as the distance increases from the surface of the composite structure to the back surface of the structure, the strength of the tension wave also decreases. The type of material also may reduce the strength of the tension wave as it travels through the composite object.
As a result, a larger compression wave is needed to produce a tension wave with the desired strength to accurately test the bonds between parts of an object. A larger compression wave requires more energy from the laser, which requires a larger and more expensive laser unit.
Using laser bond inspection also may be difficult for irregularly-shaped objects. For irregularly-shaped objects, the waves may reflect irregularly, further decreasing the strength of the reflected tension wave. Irregularly-shaped objects may also fail to properly direct tension waves to locations of interest within the structure. As a result, testing bonds in a composite object with compression waves produced by a laser may have limited application and may not produce the desired results.
Therefore, it would be desirable to have a method and apparatus that takes into account one or more of the issues discussed above, as well as possibly other issues.